1. Field of the Invention
The present invention relates to a photographic silver halide emulsion, more particularly, it relates to a silver halide photographic emulsion which is spectrally sensitized with two kinds of sensitizing dyes which have a supersensitizing effect in combination.
2. Description of the Prior Art
There are known various methods of spectrally sensitizing silver halide photographic emulsion, e.g., various methine dyes have been used for spectral sensitization. Among those, thiacarbocyanine dyes or selenacarbocyanine dyes are useful for spectrally sensitizing silver halide photographic emulsions in the red wave length region. On the other hand, a benzimidazolothiacarbocyanine dyes has its maximum spectral wave length at 590 nm to 620 nm and is therefore a useful sensitizing dye.
Where the above dyes are used alone, a sufficient supersensitizing effect cannot be obtained. A method where two kinds of sensitizing dyes are used is known as supersensitization and the super additive sensitizing effect produced therebetween brings about high spectral sensitivity. However, it is not easy to find dyes which provide a supersensitizing effect when used in combination. The slightest change in the chemical structure and a difference in substituents greatly influences the occurrence of a supersensiziting effect and a degree of the effect. Therefore, it is impossible to predict the occurrence of a sensitizing effect from chemical structure.
It is known that a combination of a thiacarbocyanine dye or a selenacarbocyanine dye and a dimethinemerocyanine dye is practically useful to panchromatically sensitize photographic light-sensitive materials in the graphic arts, particularly lithographic light-sensitive materials which yield a dot image from an original with continuous tone images using a contact screen (see U.S. Pat. No. 3,808,009 and German Patent Application (OLS) No. 2,101,071). This is because a merocyanine dye has advantageous properties such as an increased spectral sensitizing effect on an emulsion which is mainly composed of silver chloride and increased gradation (high contrast) in the preparation of the lith light-sensitive materials.
However, the above-mentioned spectral sensitizing method is insufficient for the purpose of decreasing residual color after development. Also, there are problems in the preparation of the light-sensitive materials, that is, where a solution of a merocyanine dye and a solution of a carbocyanine dye are mixed and stored, it often happens that fog is increased and sensitivity is lowered with the passage of time.